Condenser Image Planes

In a microscope optical system, the lamp filament is imaged in the focal plane of the condenser aperture diaphragm when the microscope is configured to operate under conditions of Köhler illumination. This tutorial explores the relationship between image planes relevant to the field and condenser diaphragms and how aperture size affects ray trace pathways.

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The tutorial initializes with the field and aperture diaphragms opened about 80-percent of their maximum values. Ray traces are illustrated originating from a focal point in the field diaphragm plane (Image Plane (1)), traveling through the condenser aperture diaphragm and lenses, and converging onto a focal point in the specimen plane (Image Plane (2)). The Field Diaphragm Diameter and Aperture Diaphragm Diameter sliders can be utilized to adjust the opening size of these iris diaphragms. As the sliders are moved to the right and left, changes in the diaphragm apertures affect the path of ray traces through the condenser lens system, ultimately influencing the effective working numerical aperture of specimen illumination.

In the optical train, the field diaphragm (Image Plane (1)) is imaged in the same plane as the specimen (Image Plane (2)) when the microscope is configured for Köhler illumination. The front focal plane of the condenser (F') resides in the center of the aperture diaphragm. Lengths a and b represent the distances of the field diaphragm (Image Plane (1)) and the specimen plane (Image Plane (2)) from the principal planes of the condenser lens, respectively. Light emitted by the lamphouse and passing through the condenser is formed into a cone of illumination that passes through the specimen. Adjustment of the condenser aperture iris diaphragm opening size controls the numerical aperture of this illumination cone.

Contributing Authors

Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.

John C. Long and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.